Underground storage caverns have been used for storage since the beginning of time. As early as 1940 underground storage caverns have been used to store natural gas, compressed air, hydrogen, and liquid hydrocarbons (including crude oil), amongst others. In many applications salt caverns have been man-made to hold hydrocarbons. The parameters of a storage caverns must be determined prior to use and during its lifetime since storage caverns decay and/or change shape over time.
Cavern survey logging refers to measurements of storage caverns using acoustic techniques to evaluate the storage cavern size, shape, volume, integrity and other parameters. The survey report is usually presented with the basic measurements and an estimate of cavern parameter change since the previous survey. Survey logging of fluid-filled storage caverns is time-consuming and subject to error from missing storage cavern anomalies and features difficult to characterize.
Current techniques used to measure cavern parameters are lacking. Referring to FIGS. 1a and 1b, in a prior art acoustic technique of measuring storage caverns, a single beam transducer is swept horizontally over 360 degrees at various depths or depth stations. The limited number and type of data points gathered in this technique leave significant information about the storage cavern unknown. Small target detection is difficult, if not impossible, to obtain even with readings at a plurality of depth stations. Prior art techniques are also time consuming and costly. FIG. 2 is a representation of the data obtained from a prior art cavern measuring system. Consequently, there is a need for the reliable, accurate measurement apparatus and method to determine whether and to what extent a storage cavern has undergone a change of shape, remains serviceable or requires remedial action.